Apparatus for irradiating a target volume

ABSTRACT

An irradiation apparatus for irradiating by scanning a target volume according to a predetermined dose profile with a scanning beam of charged particles forming an irradiation spot on said target volume, said apparatus comprising:
         a beam generating device,   a reference generator for computing, from said predetermined dose profile, through a dynamic inverse control strategy, the time evolution of commanded variables, these variables being the beam current I(t), the spot position settings x(t),y(t) and the scanning speed settings v x (t), v y (t),   a monitor device having means for detecting at each time (t), the actual spot position as a measured position defined by the values x m (t),y m (t) on the target volume,
 
characterised in that said irradiation apparatus further comprises means for determining the differences e x (t), e y (t) between the measured values x m (t), y m (t) and the spot position settings x(t) and y(t), and means for applying a correction to the scanning speed settings v x (t) and v y (t) depending on said differences e x (t), e y (t). The present invention is also related to a monitor for determining beam position in real-time.

FIELD OF THE INVENTION

The present invention is related to an apparatus for irradiating atarget volume with a beam of charged particles such as protons or heavyions and to a method for implementing said apparatus.

The present invention is also related to an apparatus for monitoringsaid beam.

A possible application of such apparatus is for the treatment of tumorsin patients.

STATE OF THE ART

Many types of irradiation systems are used for the treatment of tumorsin patients. It is important to precisely deliver the required dose tothe tumor region, and as little dose as possible to the environinghealthy tissues. It has been known to use X-rays or gamma rays for thispurpose. High-energy electrons have also been used. However, heaviercharged particles have the property that they deposit energy in matteraccording to the so-called “Bragg-peak”, i.e. a dose-depth curve havinga plateau, a peak, and a sharp fall-off beyond the peak. The height ofthe peak corresponding to the depth of penetration of the beam in thetissues depends on energy. It is therefore possible to direct a beam toa precise volume in a patient.

Many techniques have been devised for delivering the dose according to arequired pattern. The double scattering method combines the use of afirst and a second scatterers for producing a beam having a width largerthan the tumor to be treated, and a collimator for delimiting the beamto the exact tumor shape (CHU W. T. et al.:“Instrumentation fortreatment of cancer using proton and light-ion beams” Rev.Sci. Instrum.64(8), August 1993, pages 2080-2081).

In the so-called “voxel scanning method”, the target volume is dividedin volume elements called “voxels”. The beam is directed to a firstvoxel, and when the prescribed dose is reached, the irradiation isstopped by diverting the beam in another direction with a fast kickermagnet. A sweeper magnet is then instructed to direct the beam to a nextvoxel, and the irradiation of this voxel is performed (PEDRONI E. etal.: “200 MeV proton therapy project at the Paul Scherrer Institute:Conceptual design and practical realization”, Med. Phys. 22(1), January1995, pages 39-42).

The international application WO00/40064 in the name of the applicantdiscloses an improved technique, called “Pencil Beam Scanning”(hereafter named PBS) wherein the beam is not interrupted between theirradiation of successive individual voxels. Said method will bedescribed hereafter in details. In the PBS method, as shown on FIG. 1,the beam is moved continuously along a path, by scanning magnets in thex and y directions. The target volume is irradiated layer by layer. Witha simultaneous modulation of the beam spot speeds and variation of beamcurrent, one can obtain any dose distribution on a scanned slice and anexcellent mapping of the delivered dose to the target volume. Both thevoxel scanning method and the PBS method can perform a 3D conformalirradiation.

In the PBS method, as shown on FIG. 2, the therapist uses a treatmentplanning system for obtaining a dose map D(x,y,z) giving, for each point(x,y,z) in the target, the value of the required dose. From this dosemap, a reference generator computes the trajectories, giving, for eachdepth z, the required speeds v_(x)(t), v_(y)(t) and current I(t) of thepencil beam, as a function of time t. The word “trajectory” is used herein the sense used in control theory, i.e. the time dependence of acommanded variable. The v_(x)(t) and v_(y)(t) signals are used to drivethe x and y scanning magnets. A first inner control loop ensures thatthe magnets receive the correct control output from the scanning magnetpower supply (SMPS) derived from the reference generator calculation.The beam current setpoint I(t) is used to drive the Ion SourceElectronic Unit (ISEU), feeding the Ions source arc power supply (ArcPS). A second inner control loop ensures that the measured beam currentis according to the requirement. The beam current is measured by anionisation chamber in the beam line.

When the beam traverses matter, a scattering occurs, and the width ofthe beam is increased. For PBS, it is necessary that the beam be asnarrow as possible.

In the above scheme, however, no certainty is given that the actual doseapplied is equal to the required dose map. Many sources of noise, driftand errors may result in a discrepancy between actual and required dose.Moreover, for safety reasons, it is of the utmost importance that, incase of equipment failure, the irradiation apparatus reacts safely.Means for monitoring beam position and current in real-time, with goodprecision, over a large area, and that do not increase the beam width,and thereby allowing a fast control, have heretofore not been available.

AIMS OF THE INVENTION

The present invention aims to provide a PBS irradiation apparatuswherein the difference between the actual and required dose isminimised.

Another aim of the present invention is to provide an apparatus havingan improved safety, in case of equipment failure.

The present invention also aims to provide an irradiation apparatushaving means for monitoring the beam position and current in a preciseand rapid way, and that do not increase the beam width.

In particular, the present invention aims to provide an irradiationapparatus and process, which allow optimal 3d conformation of the dose,with improved precision and safety.

SUMMARY OF THE INVENTION

A first object of the present invention is related to an irradiationapparatus for irradiating by scanning a target volume according to apredetermined dose profile with a scanning beam of charged particlesforming an irradiation spot on said target volume, said apparatuscomprising:

-   a beam generating device,-   a reference generator for computing, from said predetermined dose    profile, through a dynamic inverse control strategy, the time    evolution of commanded variables, these variables being the beam    current I(t), the spot position settings x(t),y(t) and the scanning    speed settings v_(x)(t), v_(y)(t),-   a monitor device having means for detecting at each time (t), the    actual spot position as a measured position defined by the values    x_(m)(t),y_(m)(t) on the target volume, characterised in that said    irradiation apparatus further comprises means for determining the    differences e_(x)(t), e_(y)(t) between the measured spot values    x_(m)(t), y_(m)(t) and the spot position settings x(t) and y(t), and    means for applying a correction to the scanning speed settings    v_(x)(t) and v_(y)(t) depending on said differences e_(x)(t),    e_(y)(t).

Preferably, the monitor device further comprises means for measuring thetotal instantaneous dose deposited by the beam in the target volume, andmeans for correcting this dose deposition or for pausing the beam andinforming an external operator when said instantaneous dose is outsideof an expected range. The term “total instantaneous dose” refers to thedose deposited in a short time interval (of the order of 100microseconds) during the scanning. It is proportional to theinstantaneous beam current divided by the scanning speed.

Preferably, the irradiation apparatus according to the present inventionis arranged so as to generate a beam in one direction and so as toirradiate with said beam the target volume in layers perpendicular tosaid direction, said layers being irradiated in one or more irradiationframes, said apparatus being characterised in that the monitor devicecomprises in addition means for determining the dose distribution in aplane perpendicular to the beam direction, for each successiveirradiation frame, and means for pausing the beam and informing theexternal operator when said dose distribution is outside of an expectedrange.

A second object of the present invention is related to an apparatus formonitoring a beam of charged particles comprising:

-   a first ionisation chamber with an anode and a corresponding    cathode, said chamber having a set of parallel conducting strips on    the anode, and a conducting surface on the cathode, and a gas gap in    between,-   a second ionisation chamber with an anode and a corresponding    cathode, said chamber having a set of parallel conducting strips on    said anode, and a conducting surface on said corresponding cathode,    and a gas gap in between, the strips of the second ionisation    chamber being orthogonal to the strips of the first ionisation    chamber.

The apparatus for monitoring the beam may also comprise a thirdionisation chamber having an integral anode and an integralcorresponding cathode, and a fourth ionisation chamber having an anode,said anode being made of an array of pads, and a corresponding cathode.

In said ionisation chambers, the gap between the anode and thecorresponding cathode is preferably comprised between 3 mm and 15 mm.

In addition, in said ionisation chambers, the electric field between theplates defining the anode and the cathode may be comprised between 1 and8 kV/cm.

Preferably, the gap in the first and second ionisation chambers is 5 mm,the electric field in said ionisation chambers is 2 kV/cm or higher,while the gap in the third ionisation chamber is 10 mm, and the electricfield in said third chamber is 4 kV/cm or higher.

The invention is also related to a process for irradiation of a targetvolume with a beam of charged particles forming an irradiation spot onsaid target volume, said process comprising the steps of

-   determining a dose distribution profile referenced as a map    D(x,y,z);-   determining, from said dose map, trajectories comprising spot    position settings x(t), y(t), scanning speed settings v_(x)(t),    v_(y)(t) and a beam current setting I(t) for a set of irradiation    depths z;-   feeding said scanning speed signals v_(x)(t) and v_(y)(t) to a    scanning magnet system;-   feeding said beam current signal I(t) to a beam generating device;-   detecting and measuring the actual spot values x_(m)(t), y_(m)(t),-   determining the differences e_(x)(t), e_(y)(t) between said measured    spot values x_(m)(t), y_(m)(t) and the spot position settings x(t)    and y(t); and-   applying a correction to the scanning speed settings v_(x)(t) and    v_(y)(t) depending on said differences e_(x)(t), e_(y)(t).

Finally, the invention is also related to the use of said irradiationapparatus or process for irradiating a target.

SHORT DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general schematic representation of a PBS irradiationapparatus according to the prior art or according to the invention.

FIG. 2 is a block diagram of the control signals involved in the workingof a PBS irradiation apparatus according to the prior art.

FIG. 3 is a block diagram of the control signals involved in the workingof a PBS irradiation apparatus according to the invention.

FIG. 4 is a schematic drawing of a beam monitor for an irradiationapparatus according to the invention.

FIG. 5 is a block diagram of the control signals involved in the workingof a PBS irradiation apparatus according to a preferred embodiment ofthe invention.

FIG. 6 is a sectional side view of a beam monitor for an irradiationapparatus according to the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

A first embodiment of the present invention is illustrated on FIG. 3.

Using the treatment planning system, the therapist generates a dose mapD(x,y,z) representing the dose for each point x,y,z in the target. Thereference generator computes the trajectories, comprising the requiredspeed v_(x)(t), v_(y)(t),the required beam current I(t), and, inaddition, the required position x(t), y(t). The speed data are sent to ascanning magnet power supply (SMPS). The coil of a scanning magnetbeing, in first approximation, a pure inductance, the current throughthe coil is proportional to the time integral of the applied voltage.The displacement of the beam is proportional to the field, and hence tothe current. Therefore, by applying to the coils of the scanning magnetsa voltage proportional to the required speed v_(x)(t) or v_(y)(t), oneobtains the required beam spot speed.

According to the invention, an outer control loop for controlling thedose comprises the following elements: a monitor device performs thefunction of determining the x and y position of the beam, at a samplingrate of 5 to 10 kHz. This is performed by two orthogonal strip planes.The strips are 0.6 cm wide. The first strip plane comprises 38 strips,and the second strip plane comprises 48 strips, in a directionorthogonal to the first strips. Due to the gaussian-like shape of thebeam, one can reconstruct the mean position x, y of the beam with aprecision better than a fraction of a mm, by applying the speedcorrection as described hereafter. These measured values are subtractedfrom the required positions given by the reference generator. Thedifferences are fed into a soft gain controller, which may be aclassical PID controller, and the output of it is added, as acorrection, to the required speeds v_(x)(t), and v_(y)(t). Thisselection of acquisition resolution and frequency is attainable with themonitor described hereafter, and allows the outer control loop tocorrect efficiently any discrepancy between the required and actualdose.

The strip ionisation chambers acquire the charge collected for each ofthe strips at a rate of 10 kHz. The acquired position of the beam may bedetermined by computing the centroid of the set of acquired values. Thiswould give an excellent precision if the beam were static, or movingslowly with respect to the acquisition period. However, at a speed of 20m/s, the beam moves 2 mm during the 100 μs acquisition period, giving atoo large error. Therefore, a speed correction is applied by (i)determining the instantaneous beam speed in the successive acquiredpositions, and (ii) applying a position correction on the actualposition, corresponding to the displacement as calculated from theinstantaneous beam speed. This achieves the above-quoted precision.

Referring to FIG. 1, the beam is scanned with a regular pattern over atransversal area of the target with a beam spot characterized by agaussian fluence distribution with a sigma at isocenter in air variablefrom 2 to 10 mm. The beam spot moves with a maximum velocity of 20 m/salong the x direction and of 2 m/s in the y direction over a maximumarea of 40×30 cm² at isocenter. Thanks to the high scanning speed, thetotal dose in every plane is released by means of several subsequentscanning frames of the target area, each scanning frame corresponding tothe deposition of a few percent of the total dose. A dose deposition of0.04 Gy is chosen as the instantaneous maximum of the dose distributionin one scanned line, corresponding to 2% of a total dose of 2 Gy. Thisway, the number of scannings will be different for the different targetslabs, but the dose-rate and therefore the beam current will remainwithin a limited range on the whole target volume.

Referring to FIG. 4, the monitor comprises a set of successiveionisation chambers. The general principle of an ionisation chamber isas follows: A high voltage is applied between two parallel electrodes. Agas (here air or nitrogen) between the plates is ionised by the beampassing perpendicularly to the planes. As a result of the electricfield, the ions are collected on the electrodes, and the charge can bemeasured. As the creation of one electron-ion pair requires a knownaverage energy, depending on the gas and on the irradiation type, thecollected charge is directly proportional to the energy deposited in thegas. A recycling integrator circuit provides a 16-bit counterproportional to the detected charge. The recycling integrator wasdeveloped as a 0.8 μm CMOS technology chip (TERA06) by INFN (IstitutoNazionale di Fisica Nucleare, Torino). Each of these chips provides 64channels. The minimum detectable charge is adjustable between 50 fC and800 fC, and the read rate in the linear region can be as high as 5 MHz.The values provided by the counters are sent to an Ionisation ChamberElectronic Unit (ICEU) and the processed data are used by the MasterControl Unit (MCU) of the irradiation apparatus for performing thecontrol, safety and operator interface functions. A redundant padchamber performs a redundant check, for improving safety.

As described above, the monitor comprises two strip planes. In additionto these strip planes, the monitor may comprise an integral plane,measuring the instantaneous beam current, and hence the totalinstantaneous dose. This data is acquired at the same rate as the stripdata.

The monitor also comprises a plane made of individual square pads. Thepads have a size of 0.7 cm×0.7 cm.

FIG. 5 shows a preferred embodiment of the invention. In thisembodiment, the monitor performs two additional measurements, andadditional control functions are based thereon. The first additionalmeasurement is the measurement of the instantaneous beam current. Thismeasurement is compared with the required value, and the resulting erroris fed into the outer controller for improving the correction to speedsettings. The outer controller also performs the function of pausing thebeam (i.e. stop any irradiation) in case the error is larger than anacceptable level. This realises a first level of safety. The outercontroller can also provide an input to the reference generator formodifying the trajectory calculation. The second additional measurementis the 2D dose map, as measured by the pad plane. These are compared, ata rate up to 20 Hz, with the dose map, and can cause a beam pause incase of large error. This provides a second level of safety. A thirdsafety is provided by the use of a redundant pad monitor, performing, inparallel, the same function as the primary pad plane.

Referring to FIG. 6, the monitor is made of a main monitor comprisingsequentially a first strip plane anode, a 5 mm gap, a dual side cathode,a 5 mm gap, a second strip plane, in an orthogonal orientation, anintegral anode, a 1 cm gap, a dual side cathode, a 1 cm gap and a padplane. The redundant monitor comprises a pad anode, a 1 cm gap and acathode. The recycling integrator chips are located on the sides of theplanes. Both chambers have a metallic enclosure and thin entrance andexit window metallic foils. The enclosures and windows are grounded.Both enclosures are filled with dry air or nitrogen.

In contrast to other beam delivery methods, like the double scatteringmethod, in the PBS method, one can keep the beam in the vacuum tubealmost up to the patient. The only equipment in line with the beam isthe monitor, and this one is located at the end of a prolonged vacuumtube. The length of travel of the beam in air, which cause beamscattering, is thereby minimised. The distance between the monitor andisocenter is less than 60 cm.

1. An irradiation apparatus for irradiating by scanning a target volumeaccording to a predetermined dose profile with a scanning beam ofcharged particles forming an irradiation spot on said target volume,said apparatus comprising: a beam generating device, a referencegenerator for computing, from said predetermined dose profile, through adynamic inverse control strategy, the time evolution of commandedvariables, these variables being the beam current I(t), the spotposition settings x(t),y(t) and the scanning speed settings v_(x)(t),v_(y)(t), a monitor device having means for detecting at each time (t),the actual spot position as a measured position defined by the valuesx_(m)(t),y_(m)(t) on the target volume, wherein said irradiationapparatus further comprises means for determining the differencese_(x)(t), e_(y)(t) between the measured values x_(m)(t), y_(m)(t) andthe spot position settings x(t) and y(t), and means for applying acorrection to the scanning speed settings v_(x)(t) and v_(y)(t)depending on said differences e_(x)(t), e_(y)(t).
 2. The irradiationapparatus according to claim 1, wherein the monitor device furthercomprises means for measuring the total instantaneous dose deposited bythe beam in the target volume, and means for correcting this dosedeposition or for pausing the beam and informing an external operatorwhen said instantaneous dose is outside of an expected range.
 3. Theirradiation apparatus according to claim 1, arranged so as to generate abeam in one direction and so as to irradiate with said beam the targetvolume in layers perpendicular to said direction, said layers beingirradiated in one or more irradiation frames, wherein the monitor devicecomprises in addition means for determining the dose distribution in aplane perpendicular to the beam direction, for each successiveirradiation frame, and means for pausing the beam and informing anexternal operator when said dose distribution is outside of an expectedrange.
 4. A process implementing the apparatus as claimed in claim 1,said process comprising the steps of: determining a dose distributionprofile referenced as a map D(x,y,z); determining, from said dose map,trajectories comprising spot position settings x(t), y(t), scanningspeed settings v_(x)(t), v_(y)(t) and a beam current setting I(t) for aset of irradiation depths z; feeding said scanning speed signalsv_(x)(t) and v_(y)(t) to a scanning magnet system; feeding said beamcurrent signal I(t) to a beam generating device; detecting and measuringthe actual spot values x_(m)(t), y_(m)(t), determining the differencese_(x)(t), e_(y)(t) between said measured spot values x_(m)(t), y_(m)(t)and the spot position settings x(t) and y(t); and applying a correctionto the scanning speed settings v_(x)(t) and v_(y)(t) depending on saiddifferences e_(x)(t), e_(y)(t).
 5. Use of the apparatus according toclaim 1 or a process implementing the apparatus as claimed in claim 1,said process comprising the steps of: determining a dose distributionprofile referenced as a map D(x,y,z); determining, from said dose map,trajectories comprising spot position settings x(t), y(t), determining,from said dose map, trajectories comprising spot position settings x(t),v(t), scanning speed settings v_(x)(t), v_(y)t and a beam currentsetting I(t) for a set of irradiation depths z; feeding said scanningspeed signals v_(x)(t) and v_(y)(t) to a scanning magnet system; feedingsaid beam current signal I(t) to a beam generating device; detecting andmeasuring the actual spot values x_(m)(t), y_(m)(t), determining thedifferences e_(x)(t), e_(y)(t) between said measured spot valuesx_(m)(t), y_(m)(t) and the spot position settings x(t) and v(t); andapplying a correction to the scanning speed settings v_(x)(t) andv_(y)(t) depending on said differences e_(x)(t), e_(y)(t) forirradiating a target.